The present invention relates generally to a way to improve the transfer of water vapor produced by a fuel cell, and more particularly to an improved sealing strategy for a water vapor transfer (WVT) unit.
Fuel cell systems produce electrical energy through the oxidation and reduction of a fuel and an oxidant. Hydrogen, for example, is a very appealing fuel source because it is clean and it can be used to produce electricity efficiently in a fuel cell. The automotive industry has expended significant resources in the development of hydrogen fuel cells as a source of power for vehicles. Vehicles powered by hydrogen fuel cells would be more efficient and would generate fewer emissions than today's vehicles employing internal combustion engines.
In a typical fuel cell system, hydrogen or a hydrogen-rich gas is supplied as a reactant through a flowpath to the anode side of a fuel cell while oxygen (such as in the form of atmospheric oxygen) is supplied as a reactant through a separate flowpath to the cathode side of the fuel cell. Catalysts, typically in the form of a noble metal such as platinum, are placed at the anode and cathode to facilitate the electrochemical conversion of the reactants into electrons and positively charged ions (for the hydrogen) and negatively charged ions (for the oxygen). In one well-known fuel cell form, the anode and cathode may be made from a layer of electrically-conductive gaseous diffusion media (GDM) with the catalysts deposited thereon to form a catalyst coated diffusion media (CCDM). An electrolyte layer (also called an ionomer layer) separates the anode from the cathode to allow the selective passage of ions from the anode to the cathode while simultaneously prohibiting the passage of the generated electrons; instead, the electrons are forced to flow through an external electrically-conductive circuit (such as a load) to perform useful work before recombining with the charged ions at the cathode. The combination of the positively and negatively charged ions at the cathode results in the production of non-polluting water as a by-product of the reaction. In another well-known fuel cell form, the anode and cathode may be formed directly on the electrolyte layer to form a layered structure known as a membrane electrode assembly (MEA).
The proton exchange membrane (PEM) fuel cell has shown particular promise for vehicular and related mobile applications. The electrolyte layer of a PEM fuel cell is a solid proton-transmissive membrane, such as a perfluorosulfonic acid membrane (PFSA) (a commercial example of which is Nafion™). Regardless of whether the above MEA-based approach or CCDM-based approach is employed, the presence of an anode separated from a cathode by an electrolyte layer forms a single PEM fuel cell; many such single cells can be combined to form a fuel cell stack, increasing the power output thereof. Multiple stacks can be coupled together to further increase power output.
Fuel cells, particularly PEM fuel cells, require balanced water levels to ensure proper operation. For example, it is important to avoid having too much water in the fuel cell, which can result in the flooding or related blockage of the reactant flowfield channels, thereby hampering cell operation. On the other hand, too little hydration limits the electrical conductivity of the membrane and can lead to premature cell failure. Exacerbating the difficulty in maintaining a balance in water level is that there are numerous conflicting reactions taking place in a fuel cell that are simultaneously increasing and decreasing local and global hydration levels.
One method of ensuring adequate levels of hydration throughout the fuel cell includes humidifying one or both of the reactants before they enter the fuel cell. For example, the water produced at the cathode can be used, with an appropriate humidification device, to reduce the likelihood of dehydration of the anode or the PFSA ionomer membrane. One example of such a humidification device is a water vapor transfer (WVT) unit (also referred to as a membrane humidifier) that extracts the moisture from a humid fuel cell flowpath or flow channel and places it into a flowpath used to convey a reactant low in humidity. This is generally accomplished by using a WVT membrane that is disposed between adjacent high humidity and low humidity fluids. The membrane allows water vapor to pass through it from the higher humidity fluid to the lower humidity fluid while inhibiting the undesirable direct passage of gases from the low humidity fluid (for example, cathode inlet gases) to the outlet containing high humidity fluid (for example, cathode outlet gases) without having first passed through the fuel cell. In one form of construction, this membrane may be attached to a GDM. The combination of the WVT membrane and the GDM may be referred to as a separator, a separator plate, or a membrane humidifier assembly. Numerous such separator plates may be stacked together such that alternating layers facilitate the interaction of the dry and humid fluids.
In one form, the stacked separator plates typically include a plurality of in-plane flow channels adapted to convey the cathode and anode fluids. The WVT unit may be a cross-flow WVT unit in which the flow channels of the adjacent plates are oriented perpendicular to each other. In another configuration, the WVT unit may define a counter-flow arrangement wherein the flow through the wet plate is in the opposite direction of the flow through the dry plate.
Traditionally, the WVT unit is housed in a generally cube-shaped unit attached externally to the fuel cell stack and having four manifolds which serve as the inlets and outlets for the respective wet and dry flowpaths. Typically, the WVT unit housing is incorporated into the fuel cell system within a fuel cell module such as a lower end unit (LEU). To prevent leaking of both wet and dry fluids, which decreases the efficiency and life of the fuel cell system, the manifolds are sealed on the face through which they connect to the stack within the WVT unit. The afore-mentioned cross-flow configuration requires sealing on each plane through which the inlet and outlet manifolds are connected to the WVT unit; generally, sealing takes places on at least three planes of the WVT unit with one of these planes being at an angle to the other two. Seals are used to seal all four fluid flow manifolds (wet inlet and outlet and dry inlet and outlet) to their mating components within the LEU.
Often elastomeric seals are used to seal both the wet and the dry flows. Elastomeric seals require compression to maintain a tight seal. Maintaining adequate compression is particularly difficult in a cross-flow design where sealing must take place in two or more orthogonal directions of the WVT unit due to the perpendicular orientation of the wet and dry streams as described above. This, in turn, makes it difficult to maintain a tight seal on more than two of the four sealing planes. Traditional sealing strategies do not allow for sealing on fewer than three planes with one of those planes being perpendicular to the other two.
Once sealed, the WVT unit must also be able to accommodate cell expansion and contraction based on changing hydration levels and temperature. As such, managing thermal and humidity related expansion and contraction (up to 5 mm total displacement) has been an issue with earlier WVT unit stacked plate designs. The movement can lead to breakdown of sealing interfaces causing the undesirable dry and wet flow leakages discussed above. Coil springs may be used to place end plates, disposed on either end of the WVT unit, under tension. The springs hold the plates together while still allowing for expansion and contraction of the core through the spring coils, but do so with additional weight and complexity.